The invention pertains to handheld dispensing and delivery apparatus for fluent materials. More particularly, this invention relates to material injectors and guns, such as bone cement injectors and caulk guns.
Reconstructive bone surgery is a common procedure to replace bone structures. For example, replacement hip surgery uses a prosthetic hip joint to replace a patient""s hip joint. Such surgical procedures include removing the hip joint from a femur, packing bone cement within an inner canal of the femur, and positioning the prosthetic hip joint into the bone cement inside the femur. Subsequently, the cement is allowed to harden, thereby securing the hip joint in place.
Bone cement is typically pre-mixed, then delivered to a surgical site while in a fluent state. Accordingly, such bone cement comprises a fluent material. Such fluent material can vary in viscosity. Furthermore, the viscosity of such fluent material can vary over time in the case of bone cement, as the bone cement begins to set or harden during delivery. Accordingly, it is desirable to adjust delivery force or pressure with which a fluent material can be delivered from a hand-held dispensing and delivery apparatus, such as a bone cement injector.
U.S. Patent No. 5,431,654 discloses a bone cement injector having a trigger structure that cooperates with a low force pawl and a high force pawl in order to change actuation force from a discrete, relatively low force to a discrete, relatively high force. Such force change is carried out by changing (or switching) the pawl that is engaged by the trigger structure. However, no provision is made for infinitely adjusting the fulcrum force of a pawl, nor for variably adjusting the fulcrum distance for a single driving pawl. Hence, improvements are needed over the prior art.